The present invention relates to a digital cassette tape reproducing device and, particularly, to an improvement of a digital compact cassette tape reproducing device having a digital signal reproducing function of reproducing a digital audio signal recorded on a magnetic tape and an analog signal reproducing function of reproducing an analog audio signal recorded on a magnetic tape. The invention further relates to such a compact cassette tape reproducing device having a function of recording a digital and an analog audio signal on magnetic tapes in addition to the reproducing functions, while improving tone quality of a reproduced audio signal.
A compact disc (CD) of 12 cm size has become popular as a recording medium for digital audio signals and, in order to reproduce such digital audio signals therefrom, a digital cassette tape reproducing device has been used in various audio devices such as, for example, a CD radio cassette recorder, a CD stereo component device and a car-mounted audio device.
On the other hand, in order to digitally record an audio signal on a magnetic tape and reproduce it therefrom, the so-called DAT standard has been proposed and used practically to some extent. However, a DAT recording/reproducing device has not become popular so far.
The so-called digital compact cassette tape recorder in which an audio signal is digitally recorded on a conventional cassette tape for analog recording has been proposed besides the above-mentioned techniques. Such a digital compact cassette tape recorder (referred to as a DCC player, hereinafter) functions to read a digital audio signal recorded on a cassette tape and to reproduce it as an audio signal, and to read an analog signal recorded according to a conventional standard and reproduce it as it is. In other words, the DCC player has both a digital signal reproducing function and an analog signal reproducing function with advantages of digital recording (for example, improved performance and function of hardware), and advantages of analog recording (for example, the ability of using a large volume of existing software).
FIG. 4 is a block circuit diagram of a conventional portable DCC player to be used with a headphone speaker connected thereto.
The portable DCC player in FIG. 4 comprises a head unit 1, an analog signal reproducing circuit 2, a digital signal reproducing circuit 3, a switch 5, a drive circuit 6, a headphone jack 8 and the headphone speaker 9 to be connected to the player in use.
The head unit 1 includes a magnetic head 1a for reproducing a digital signal and a magnetic head 1b for reproducing an analog signal. The head 1a functions to read an audio signal which is usually 8 to 10 bits long and recorded in a digital manner on a cassette tape (not shown) and to output a digital audio signal D. The head 1b functions to read an audio signal recorded on a cassette tape according to the conventional standard and to output it as an analog audio signal H.
The analog signal reproducing circuit 2 includes an equalizer circuit composed of an amplifier 2a and a feedback circuit EQ, etc., for correction of signal distortion which may result from variations of magnetic characteristics of a recording medium. The analog signal reproducing circuit 2 receives the analog signal H from the head 1b, produces an analog reproduced signal I by correction of its distortion and outputs it to the switch 5.
The digital signal reproducing circuit 3 includes a digital signal processing circuit 3b responsive to the digital signal D from the head 1a for producing a digital signal E, a digital-to-analog (D/A) converter portion 3a for converting the digital audio signal D into an analog audio signal F and an attenuator 4 for attenuating the analog audio signal F to obtain an analog audio signal G. The digital signal processing circuit 3b operates to demodulate the digital signal D to obtain an original digital value prior to recording, to digitally process the signal to correct errors etc., and to produce the digital signal E. The D/A converter portion 3a converts the digital signal E into an analog signal, removes noise caused by quantization by means of a low-pass filter and produces the analog reproduced signal F. The attenuator 4 produces the reproduced signal G by attenuating the amplitude level of the reproduced signal F. The attenuation is necessary to make an amplitude level of the output signal G of the digital signal reproducing circuit 3 correspond with an amplitude level of an output signal I of the analog signal reproducing circuit and, hence, an amplitude level of an input signal J from such as a tuner.
Although the switch 5 is shown in this example as a slide type switch, a switch of any other type may be used so long as it can select either the signal G or I, together with another signal, such as J, and output the selected one, G or I, to the drive circuit 6 as a reproduced signal K.
The drive circuit 6 includes a power amplifier 6a which amplifies the signal K to obtain a signal M which is supplied through a coupling capacitor 6c to the headphone jack 8 as a drive signal N by which the headphone speaker 9 connected to the headphone jack 8 is driven.
The conventional DCC player including the analog signal reproducing circuit 2 and the digital signal reproducing circuit 3 as mentioned above reproduces an audio signal recorded in an analog manner on any existing tape by means of the analog signal reproducing circuit, and reproduces a digital audio signal of high tone quality by the digital signal reproducing circuit. Since one of these signals is selected by the switch 5, the signal supplied to the headphone speaker is either an analog-recorded or a digital-recorded audio signal.
The function of the attenuator 4 of the digital signal reproducing circuit 3 in such a conventional DCC player was described briefly. In order for a better understanding of the present invention, the function of the attenuator 4 will be described in more detail. The amplitude level of the digital signal F obtained from a recorded audio signal having the usual level is usually higher than that of the analog signal I by about 17 dB and, therefore, the signal F is attenuated correspondingly so that either of the both signals to be supplied selectively to the headphone speaker has substantially the same signal level. With this scheme, the circuit portion following the drive circuit 6 can be used commonly by these signals, making the circuit construction compact.
It is clear that, by attenuating the signal F by about 17 dB, a portion of the signal F may be lost. That is, such a partial loss of signal causes the S/N ratio thereof to be degraded, which makes the high tone quality of audio signal reproduced from digital-recorded signal nonsense, and it is impossible to effectively utilize the merit of the digital recording system since a compact construction has a higher priority in a portable DCC player.